Figure 3. Magma seeps up from deep within the mantle to form the East Pacific Rise (shown in cross section along the crest of the ridge). Investigators speculate that partially melted rock from depths of 30,000 to 60,000 meters percolates upward and is produced in greater quantities in some areas (dark red) than in others (light red). They propose that the molten rock fills and expands magma chambers. Seismic measurements suggest that the tops of the chambers are at the depth indicated by the broken red line. Molten rock ascends from the magma chamber through cracks in the crust and then solidifies or erupts onto the ocean floor. The depth of the ridge (black line at top) was determined from sonar measurements. The chamber breaks below discontinuities of order one, two and sometimes three.